When an object fills or otherwise blocks the outer portion of a person's ear canal, the person perceives “hollow”, “muffled” or “booming” sounds of its own voice. This phenomenon is known as the occlusion effect. It is caused by an altered balance between air-conducted and bone-conducted transmission to the human ear. When the ear canal is open, vibrations caused by talking or chewing normally escape through the open ear canal. When the ear canal is blocked, i.e. occluded, the vibrations are instead reflected back towards the eardrum. In fact, vibrations in cartilaginous tissue and bone around the ear canal may result in build-up of relatively high sound pressure levels at the eardrum. In acoustical terms, the occluded ear canal defines a small enclosed volume which has a large acoustic impedance. The acoustic impedance is the ratio of acoustic pressure to acoustic volume velocity. Thus, when the acoustic impedance is large, small changes in volume velocity in the ear canal will generate relatively large pressure variations at the eardrum. This means that an occlusion of the ear canal results in an increase of bone-conducted sound vibrations at the ear drum compared to a non-occluded ear. When the ear is not occluded, the acoustic impedance is instead dominated by the much smaller radiation impedance from the open ear canal to the surroundings. Compared to a completely open ear canal, the occlusion effect may boost low frequency (e.g. below 500 Hz) sound pressure in the ear canal by 20 dB or more.
It may be particularly desirable to counteract the occlusion effect in implements that may require the user to speak while wearing the implement, e.g. various ear- or head-mounted devices for use during telephone conversations, as well as hearing aids.
In the field of telephony, it is known to introduce an electric “sidetone path” in the telephone handset, by instantly feeding the signal from the microphone to the speaker in the earpiece of the handset. This technique will allow the talker to hear his own voice even if the ear is occluded by the earpiece. The provision of an electric sidetone path may restore the lost air-conducted path but does not completely address or counteract the occlusion effect, which generally results in an undesired and unpleasant distortion of the sound perceived by a person when talking, specifically by the balance between low, mid and high frequencies differing from the normal situation of a non-occluded ear.
A hearing aid generally defines an earpiece that occludes the outer end of the ear canal. The hearing aid includes a microphone arranged to pick up sound in the surroundings of the hearing aid, a speaker arranged to generate audible sound in the ear canal, and electronic circuitry configured to amplify and otherwise modify the signal produced by the microphone for supply to the speaker. Thus, the hearing aid also produces an electric sidetone path that allows the bearer to hear his own voice even if the ear is occluded by the hearing aid. For the bearer of a hearing aid, the occlusion effect may introduce an undesired and unpleasant distortion of the sound perceived by the bearer when talking.
The most common approach of dealing with the occlusion effect is to intentionally design the earpiece that occludes the ear with dedicated leaks to its surrounding. These intentional leaks may be implemented as one or more vents or through-holes in the earpiece, which are carefully designed to shunt a portion of low-frequency signals to the environment, removing all or part of the disturbing low-frequency own-voice elements. Examples of vent designs in the field of hearing aids are given the article “Occlusion effect of earmolds with different venting systems”, by Kiessling et al, published in Journal of the American Academy of Audiology, 16, 237-249 (2005).
One disadvantage of this approach is that the vent provides an acoustic path for noise to leak into the ear from the environment. The vent may also reduce the bass sensitivity of the occluded ear.
It is a general desire to counteract or reduce the occlusion effect caused by implements that occlude one or both ears of a person. This applies not only to implements that are naturally used by the person while talking, such as telephone headsets and hearing aids, but also to other implements which may be used while talking and in which the amplification of bone-conducted sound by the occlusion effect is undesired and may interfere with the user experience, e.g. in ear- or head-mounted devices for audio (e.g. music) playback, and ear- or head-mounted devices for hearing protection.